Co/Ni multilayers with improved out-of-plane anisotropy for magnetic device applications

ABSTRACT

A MTJ for a spintronic device includes a thin seed layer that enhances perpendicular magnetic anisotropy (PMA) in an overlying laminated layer with a (Co/Ni) n  composition or the like where n is from 2 to 30. The seed layer is preferably NiCr, NiFeCr, Hf, or a composite thereof with a thickness from 10 to 100 Angstroms. Furthermore, a magnetic layer such as CoFeB may be formed between the laminated layer and a tunnel barrier layer to serve as a transitional layer between a (111) laminate and (100) MgO tunnel barrier. There may be a Ta insertion layer between the CoFeB layer and laminated layer to promote (100) crystallization in the CoFeB layer. The laminated layer may be used as a free layer in a MTJ. Annealing between 300° C. and 400° C. may be used to further enhance PMA in the laminated layer.

This is a divisional application of U.S. patent application Ser. No.14/032,593 filed on Sep. 20, 2013 that is a divisional of Ser. No.13/068,398 filed on May 10, 2011 and now issued as U.S. Pat. No.8,541,855, which are herein incorporated by reference in their entirety,and assigned to a common assignee.

FIELD OF THE INVENTION

The invention relates to a magnetic device that employs a thin film madeof a Ni/Co laminate with a magnetization direction which isperpendicular to the plane of the film (perpendicular magneticanisotropy or PMA) wherein PMA is enhanced by an improved seed layerthat induces a strong (111) crystal structure in the Ni/Co multilayerstack.

BACKGROUND OF THE INVENTION

Magnetoresistive Random Access Memory (MRAM), based on the integrationof silicon CMOS with magnetic tunnel junction (MTJ) technology, is amajor emerging technology that is highly competitive with existingsemiconductor memories such as SRAM, DRAM, and Flash. Similarly,spin-transfer (spin torque or STT) magnetization switching described byC. Slonczewski in “Current driven excitation of magnetic multilayers”,J. Magn. Magn. Mater. V 159, L1-L7 (1996), has stimulated considerableinterest due to its potential application for spintronic devices such asspin-torque MRAM on a gigabit scale.

Both MRAM and STT-MRAM may have a MTJ element based on a tunnelingmagneto-resistance (TMR) effect wherein a stack of layers has aconfiguration in which two ferromagnetic layers are separated by a thinnon-magnetic dielectric layer. The MTJ element is typically formedbetween a bottom electrode such as a first conductive line and a topelectrode that is a second conductive line at locations where the topelectrode crosses over the bottom electrode. A MTJ stack of layers mayhave a bottom spin valve configuration in which a seed layer, ananti-ferromagnetic (AFM) pinning layer, a ferromagnetic “pinned” layer,a thin tunnel barrier layer, a ferromagnetic “free” layer, and a cappinglayer are sequentially formed on a bottom electrode. The pinned orreference layer has a magnetic moment that is fixed in the “y”direction, for example, by exchange coupling with the adjacentanti-ferromagnetic (AFM) layer that is also magnetized in the “y”direction. The free layer has a magnetic moment that is either parallelor anti-parallel to the magnetic moment in the pinned layer. The tunnelbarrier layer is thin enough that a current through it can beestablished by quantum mechanical tunneling of conduction electrons. Themagnetic moment of the free layer may change in response to externalmagnetic fields and it is the relative orientation of the magneticmoments between the free and pinned layers that determines the tunnelingcurrent and therefore the resistance of the tunneling junction. When asense current is passed from the top electrode to the bottom electrodein a direction perpendicular to the MTJ layers, a lower resistance isdetected when the magnetization directions of the free and pinned layersare in a parallel state (“0” memory state) and a higher resistance isnoted when they are in an anti-parallel state or “1” memory state.

As the size of MRAM cells decreases, the use of external magnetic fieldsgenerated by current carrying lines to switch the magnetic momentdirection becomes problematic. One of the keys to manufacturability ofultra-high density MRAMs is to provide a robust magnetic switchingmargin by eliminating the half-select disturb issue. For this reason,the spin torque MRAM was developed. Compared with conventional MRAM,spin-torque MRAM has an advantage in avoiding the half select problemand writing disturbance between adjacent cells. The spin-transfer effectarises from the spin dependent electron transport properties offerromagnetic-spacer-ferromagnetic multilayers. When a spin-polarizedcurrent transverses a magnetic multilayer in a CPP configuration, thespin angular moment of electrons incident on a ferromagnetic layerinteracts with magnetic moments of the ferromagnetic layer near theinterface between the ferromagnetic and non-magnetic spacer. Throughthis interaction, the electrons transfer a portion of their angularmomentum to the ferromagnetic layer. As a result, spin-polarized currentcan switch the magnetization direction of the ferromagnetic layer if thecurrent density is sufficiently high, and if the dimensions of themultilayer are small. The difference between a spin-torque MRAM and aconventional MRAM is only in the write operation mechanism. The readmechanism is the same.

For MRAM and spin-torque MRAM applications, it is often important totake advantage of PMA films with a large and tunable coercivity field(Hc) and anisotropy field (Hk). For example, PMA films may be used as apinned layer, free layer, or dipole (offset compensation) layer in a MTJelement or in PMA media used in magnetic sensors, magnetic data storage,and in other spintronic devices. Furthermore, a critical requirement isthat Hc, Hk, and other properties such as the magnetoresistive (MR)ratio do not deteriorate during processing at elevated temperatures upto the 300° C. to 400° C. range. In some applications, it is alsonecessary to limit the overall thickness of the PMA layer andunderlayers, and use only materials that are compatible with devicedesign and processing requirements.

Materials with PMA are of particular importance for magnetic andmagnetic-optic recording applications. Spintronic devices withperpendicular magnetic anisotropy have an advantage over MRAM devicesbased on in-plane anisotropy in that they can satisfy the thermalstability requirement and have a low switching current density but alsohave no limit of cell aspect ratio. As a result, spin valve structuresbased on PMA are capable of scaling for higher packing density which isone of the key challenges for future MRAM applications and otherspintronic devices.

When the size of a memory cell is reduced, much larger magneticanisotropy is required because the thermal stability factor isproportional to the volume of the memory cell. Generally, PMA materialshave magnetic anisotropy larger than that of conventional in-plane softmagnetic materials such as NiFe or CoFeB. Thus, magnetic devices withPMA are advantageous for achieving a low switching current and highthermal stability.

Several PMA material systems have been proposed and utilized in theprior art such as multilayers of Pt/Fe, Pd/Co, and Ni/Co, and ordered(e.g., L10 structures) as well as unordered alloys but there is still aneed for improvement in Hc, Hk, temperature stability, and materialcompatibility. There is a report by M. Nakayama et al. in “Spin transferswitching in TbCoFe/CoFeB/MgO/CoFeB/TbCoFe magnetic tunnel junctionswith perpendicular magnetic anisotropy”, J. Appl. Phys. 103, 07A710(2008) related to spin transfer switching in a MTJ employing a TbCoFePMA structure. However, in a MTJ with a TbCoFe or FePt PMA layer,strenuous annealing conditions are usually required to achieve anacceptably high PMA value. Unfortunately, high temperatures are not sopractical for device integration.

Although (Co/Pt)_(X) laminates are capable of generating high PMA, Co/Ptand similar multilayers including Co/Pd and Co/Ir and alloys thereofsuch as CoCrPt are not always desirable as a PMA layer in a MTJ elementbecause Pt, Pd, Ir, and Cr are severe spin depolarizing materials andwill seriously quench the amplitude of spintronic devices.

Among the PMA material systems studied, a Ni/Co multilayer is one of themore promising candidates because of its large potential Hc and Hk, goodstability at high anneal temperatures, and potential compatibility withother materials used in magnetic devices. However, Ni/Co multilayerstypically require a thick seed layer to induce high PMA. A 500 AngstromTi or 500 Angstrom Cu seed layer with heating to 150° C. is used by P.Bloemen et al. in “Magnetic anisotropies in Co/Ni (111) multilayers”, J.Appl. Phys. 72, 4840 (1992). W. Chen et al. in “Spin-torque drivenferromagnetic resonance of Co/Ni synthetic layers in spin valves”, Appl.Phys. Lett. 92, 012507 (2008) describe a 1000 Angstrom Cu/200 AngstromPt/100 Angstrom Cu composite seed layer. The aforementioned seed layersare not practical with Ni/Co multilayer PMA configurations in spintronicdevices. Typically, there is a space restriction in a directionperpendicular to the planes of the spin valve layers in advanced devicesin order to optimize performance. Seed layers thicker than about 100Angstroms will require thinning a different layer in the spin valvestructure to maintain a certain minimum thickness for the MTJ elementwhich can easily lead to performance degradation.

A [(Co/Ni)₂₀] laminated structure with a thin Ta/Ru/Cu seed layer isdisclosed as a hard bias layer in U.S. Patent Application Publication2010/0330395 and as a reference layer in U.S. Patent ApplicationPublication 2009/0257151. However, even higher Hc is desirable to becompetitive with Hc values obtained with Pt/Co and Pd/Co laminates.

In U.S. Pat. No. 7,843,669, a fcc (111) crystal orientation is describedas desirable for a pinned layer or free layer but a Ni/Co laminate with(111) orientation is not disclosed.

U.S. Pat. No. 7,190,613 describes a fixed layer having a high coerciveforce and made of ordered alloys such as FePt, CoPt, and FePd, ordisordered alloys including Co/Cr, Co/Pt, Co/Cr/Pt and the like. Forordered alloys with a fct(001) orientation, an underlayer such as MgO,Pt, Pd, Au, Ag, Al or Cr with a similar crystal structure is preferred.

An improved seed layer is still needed that is thin enough to becompatible with spintronic devices, can induce high PMA in overlyingCo/Ni multilayer structures, and is compatible with the design andprocessing requirements of magnetic devices.

SUMMARY OF THE INVENTION

One objective of the present invention is to provide a seed layer/PMAlayer configuration for a magnetic device that has higher Hk and Hc thanpreviously realized and with thermal stability up to 400° C. processtemperatures.

A second objective of the present invention is to provide a material setfor a high PMA structure according to the first objective that iscompatible with other layers in the magnetic device and has a seed layerthickness of about 100 Angstroms or less.

According to one embodiment, these objectives are achieved with amagnetic element that is a MTJ having a bottom spin valve configurationin which a seed layer, PMA reference layer, tunnel barrier, free layer,and capping layer are sequentially formed on a substrate. The seed layer(underlayer) is preferably NiCr, NiFeCr, Hf, or a composite with aHf/NiCr, Hf/NiFeCr, NiFeCr/Hf, or NiCr/Hf configuration that induces astrong (111) texture in the overlying Co/Ni multilayer stack within thereference layer. Preferably, the reference layer has a[(Co/Ni)_(n)/CoFeB] configuration where n is from 2 to 30 and the CoFeB(or CoFe) layer serves as a transitional layer between the (111)crystalline structure of the Co/Ni multilayer stack and the (100)texture of a MgO tunnel barrier. The transitional layer preferably hasPMA and a magnetization aligned in the same direction as the PMA layer.The free layer may be comprised of CoFeB, CoFe, or a combinationthereof. Thus, a high MR ratio is achieved together with high PMA in thereference layer to enable greater thermal stability in the magneticelement.

In a second embodiment, the MTJ has a top spin valve structure wherein aseed layer, PMA free layer, tunnel barrier, reference layer, and cappinglayer are sequentially formed on a substrate. The PMA free layer may bea composite with a Co/Ni laminate formed on the seed layer and an uppermagnetic layer contacting the tunnel barrier. Again, the magnetic layermay be CoFeB and serve as a transitional layer between a (100) MgOtunnel barrier and a Co/Ni multilayer with a (111) texture. Both of thefirst and second embodiments may further comprise a Ta insertion layerbetween the Co/Ni multilayer and the transitional layer to preventpremature crystallization of the transitional layer before the tunnelbarrier is fabricated.

In a third embodiment, a dipole layer with an underlayer/PMA layerconfiguration as defined in the first embodiment is used to provide anoffset field to an adjacent free layer. The MTJ has a stack representedby seed layer/reference layer/tunnel barrier/free layer/spacer/dipolelayer/capping layer. The spacer may be a non-magnetic Ta layer to getteroxygen from the free layer.

Once all the layers in the MTJ stack are laid down, a high temperatureannealing of about 350° C. may be employed to increase the PMA withinthe Co/Ni laminated portion of the reference layer, free layer, ordipole layer.

Alternatively, the (Co/Ni)_(n) multilayer in the previous embodimentsmay be replaced by a CoFe/Ni, Co/NiFe, Co/NiCo, CoFe/NiFe, or aCoFe/NiCo laminated structure, or may be one of Co/Pt, Co/Pd, Fe/Pt orFe/Pd.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a magnetic element including acomposite reference layer with a laminated PMA layer/magnetictransitional layer formed according to a first embodiment of the presentinvention.

FIG. 2 is the magnetic element in FIG. 1 further comprising a Tainsertion layer between the laminated PMA layer and the magnetictransitional layer according to another embodiment of the presentinvention.

FIG. 3 is cross-sectional view of a magnetic element including acomposite free layer formed according to a second embodiment of thepresent invention.

FIG. 4 is a cross-sectional view of a magnetic element including acomposite dipole layer formed according to a third embodiment of thepresent invention.

FIG. 5 is a cross-sectional view depicting an embodiment wherein areference layer according to the first embodiment is formed in a domainwall motion device.

FIG. 6a is a plot illustrating magnetic properties measuredperpendicular to the film plane for Co/Ni multilayers grown on less thanoptimal seed layers.

FIG. 6b is a plot illustrating magnetic properties measuredperpendicular to the film plane for Co/Ni multilayers grown on Hf orNiCr seed layers according to an embodiment of the present invention.

FIGS. 7a, 7b are plots showing magnetic properties measuredperpendicular to the film plane and in-plane, respectively, for Co/Nimultilayers grown on Hf/NiCr or NiCr/Hf seed layers according to anembodiment of the present invention.

FIG. 8 shows MH curves measured in a direction perpendicular to the filmplane for a MTJ element comprised of a (Co/Ni)₁₀ laminated referencelayer formed on a Hf/NiCr seed layer.

FIG. 9 shows MH curves measured in a direction perpendicular to the filmplane for a MTJ element comprised of a (Co/Ni)₁₀ laminated dipole layerformed on a NiCr seed layer according to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a composite structure with a seed layer/PMAlayer configuration in a magnetic element wherein the seed layer inducesa strong (111) crystalline structure in an overlying (Ni/Co)_(n)multilayer thereby generating high PMA in the laminated stack. Note that“seed layer” may be used interchangeably with the term “underlayer” inthe exemplary embodiments, and (Ni/Co)_(n) and (Co/Ni)_(n) are usedinterchangeably when referring to a laminated stack. Although onlybottom and top spin valve structures are depicted in the drawings, thepresent invention also encompasses dual spin valves having an enhancedPMA layer that is incorporated in one or more of a reference layer, freelayer, dipole layer, or pinned layer in MRAM, spin-torque-MRAM, domainwall motion devices, and in other spintronic devices. Furthermore, thePMA structure may be used as a medium in magnetic sensors or in magneticdata storage applications.

A key feature of the present invention is a (Co/Ni)_(n) multilayerstructure having PMA where the perpendicular magnetic anisotropy of theaforementioned laminate arises from spin-orbit interactions of the 3dand 4s electrons of Co and Ni atoms. Such interaction causes theexistence of an orbital moment which is anisotropic with respect to thecrystal axes which are in (111) alignment, and also leads to analignment of the spin moment with the orbital moment. PMA character isenhanced by the presence of an appropriate seed layer (underlayer) alsohaving a (111) texture. Ideally, the seed layer has a composition whichis compatible with other materials in a magnetic element that containsthe (Co/Ni)_(n) laminate, is compatible with processing temperatures upto about 400° C., and is thin enough so as not to adversely affect theperformance of the magnetic element.

Referring to FIG. 1, a seed layer/PMA multilayer configuration is shownas part of a MTJ 20 with a bottom spin valve structure according to afirst embodiment of the present invention. Each of the layers in the MTJis formed in an (x, y) plane and each have a thickness in a z-axisdirection. A substrate 21 is provided that may be a bottom electrodelayer, for example, made of Ta or other conductive layers. Substrate 21may be formed on a substructure (not shown) that includes dielectric andconductive layers as well as transistors and other devices. A keyfeature is the seed layer 22 formed on substrate 21. The seed layer 22has a thickness from 10 to 300 Angstroms, and preferably 10 to 100Angstroms. According to one embodiment, the seed layer is NiCr, NiFeCr,or Hf. For a NiCr or NiFeCr seed layer, Cr content is between 35 and 45atomic % Cr, and preferably is 40 atomic % Cr. Alternatively, seed layer22 may be a composite with a Hf/NiCr, Hf/NiFeCr, NiFeCr/Hf, or NiCr/Hfcomposition wherein the NiCr or NiFeCr layer has a greater thicknessthan the Hf layer. As a result of the preferred seed layer composition,a (111) texture is induced and fully established in a (Co/Ni)_(n)multilayer or the like grown on a top surface of the seed layer. Topsurface in this context is a surface facing away from substrate 21.

Above the seed layer 22 is a composite reference layer 30 that accordingto one embodiment has a (Co/Ni)_(n)/CoFeB configuration wherein a lowerPMA layer 23 has a (Co/Ni)_(n) composition in which n is from 2 to 30,and preferably between 4 and 10. PMA layer 23 contacts the seed layerwhile a magnetic layer 24 made of CoFeB, CoFe, or a combination thereofis formed as an interface between the PMA layer and the tunnel barrier25. Each of the plurality of Co layers in the PMA layer has a thicknessfrom 1.5 to 4 Angstroms, and each of the plurality of Ni layers has athickness from 4 to 10 Angstroms.

In another embodiment, the lower PMA layer 23 may be comprised of twometals, a metal and an alloy, or two alloys having an (A1/A2)_(n)configuration where Al is a first metal or alloy selected from one ormore of Co, Ni, and Fe that may be doped with B up to 50 atomic %, A2 isa second metal or alloy selected from one or more of Co, Fe, Ni, Pt, andPd, n is the number of laminates in the (Al/A2)_(n) stack, and Al isunequal to A2. It should be understood that the laminated (A1/A2)_(n)stack has intrinsic PMA and the seed layer 22 is employed to enhance thePMA property. Thus, the PMA layer 23 may be comprised of (CoFe/Ni)_(n),Co/NiFe)_(n), (Co/NiCo)_(n), (CoFe/NiFe)_(n), or (CoFe/NiCo)_(n)laminates, for example. Alternatively, the PMA layer may have a(Co/Pt)_(n), Co/Pd)_(n), (Fe/Pt)_(n), or (Fe/Pd)_(n) composition, or acombination of the aforementioned laminates.

In yet another embodiment, the lower PMA layer 23 may be comprised oftwo individual magnetic layers separated by a non-magnetic spacer (C)providing anti-ferromagnetic (RKKY) coupling between the two magneticlayers as in an A1/C/A2 configuration. In this embodiment, thenon-magnetic spacer is preferably Ru with a thickness of 3 to 20angstroms.

The present invention also encompasses an embodiment wherein the lowerPMA layer 23 may be an alloy with a L10 structure of the form MT whereinM is Rh, Pd, Pt, Ir, or an alloy thereof, and T is Fe, Co, Ni or alloythereof. Furthermore, the MT alloy may be doped with B up to 40 atomic%.

Preferably, the magnetic layer 24 has a thickness from about 6 to 14Angstroms that is sufficiently thin to enable interfacial perpendicularanisotropy to dominate the in-plane shape anisotropy field and therebygenerate PMA character therein. According to one embodiment, PMA withinmagnetic layer 24 is achieved as a result of the interface between athin CoFeB (or CoFe) layer and a metal oxide layer in tunnel barrier 25that leads to a significant amount of interfacial perpendicularanisotropy, and the magnetic moments of layers 23, 24 are aligned in thesame direction along the z-axis. The magnetic layer serves as atransitional layer between the (111) texture in PMA layer 23 and a (100)texture in tunnel barrier 25 and may also enhance the magnetoresistive(MR) ratio. As the magnetic layer thickness becomes closer to 6Angstroms, PMA character is maximized, and as layer 24 thicknessapproaches 14 Angstroms, MR ratio is increased. Therefore, the thicknessof the magnetic layer may be adjusted between 6 and 14 Angstroms to tuneboth PMA magnitude and MR ratio.

There is a tunnel barrier layer 25 preferably made of MgO formed on thecomposite reference layer 30. However, other tunnel barrier materialssuch as AlOx, TiOx, and ZnOx may be employed. A MgO tunnel barrier layermay be fabricated by depositing a first Mg layer on the magnetic layer24, then performing a natural oxidation (NOX) process, and finallydepositing a second Mg layer on the oxidized first Mg layer. During asubsequent annealing process, the second Mg layer is oxidized to afforda substantially uniform MgO layer. If a low RA (resistance×area) valueis desired, the thickness and/or oxidation state of the tunnel barrier25 may be reduced as appreciated by those skilled in the art.

A free layer 26 is formed on the tunnel barrier layer 25 and may be acomposite and comprised of the same composition as in magnetic layer 24.Preferably, CoFeB, CoFe, or other materials which produce a combinationof high MR ratio, good switching property, and low magnetostriction areselected as the free layer.

The uppermost layer in the spin valve stack is a capping layer 27. Inone aspect, the capping layer is a composite with a lower layer 27 amade of Ta and an upper layer 27 b that is Ru which is used to provideoxidation resistance and excellent electrical contact to an overlyingtop electrode (not shown). A substantial reduction in critical currentdensity (Jc) occurs in STT-MRAM applications when a thin Ru layer isemployed as a capping layer due to the strong spin scattering effect ofRu. The Ta layer may be included to offer etch resistance in subsequentprocessing steps. Optionally, other capping layer configurations may beemployed. For example, the capping layer 27 may be a single layer of Taor Ru, a composite with a Ru/Ta/Ru configuration, or a composite with alower oxide or nitride layer and an upper Ru layer such as MgO/Ru orAlOx/Ru. An oxide as the lower layer in the capping layer may beadvantageously used to promote PMA in a thin free layer 26.

Referring to FIG. 2, the composite reference layer 30 of the firstembodiment is modified to include a Ta insertion layer 35 about 0.5 to 3Angstroms thick, and preferably 1.5 Angstroms thick, sandwiched betweenthe PMA layer and an amorphous CoFeB magnetic layer 24 to preventcrystallization of the CoFeB layer before a MgO tunnel barrier 25 isformed thereon. As a result, crystallization of an amorphous CoFeB layerduring a subsequent annealing step is driven by the (100) MgO tunnelbarrier and a major portion (not shown) of the magnetic layercrystallizes in a (100) state to maximize the MR ratio in the MTJ. Itshould be understood that a thin region (not shown) of a CoFeB magneticlayer 24 which adjoins the PMA layer 23 will have a (111) crystalstructure or remains amorphous but the thickness of this thin region maybe minimized by including the Ta insertion layer 35.

Referring to FIG. 3, a second embodiment is depicted that shows a MTJ 40wherein the seed layer/PMA layer configuration of the present inventionis incorporated in a composite free layer. In other words, the seedlayer/PMA layer configuration as defined herein is not limited to areference (pinned) layer but may be used as any functional or passivelayer in a magnetic stack. Preferably, the MTJ in this embodiment has atop spin valve structure in which a seed layer, composite free layer,tunnel barrier, reference layer, and a capping layer are sequentiallyformed on substrate 21. The seed layer 22 and PMA layer 23 are retainedfrom the first embodiment but in this case the laminated PMA layer isincluded in a composite free layer. Reference layer 29 formed on thetunnel barrier is comprised of CoFeB, CoFe, or combinations thereof andmay have a well known synthetic anti-parallel (SyAP) configuration (notshown) wherein two ferromagnetic layers such as CoFeB are separated by acoupling layer which is Ru, for example. Tunnel barrier 25 and cappinglayer 27 are retained from the first embodiment.

Preferably, the tunnel barrier is MgO to maximize the MR ratio in theMTJ 40. Furthermore, the composite free layer 31 may have a seed layer22/PMA layer 23/magnetic layer 24 configuration where the magnetic layerserves as a transitional layer between the (100) texture in the tunnelbarrier layer and the (111) crystal structure in PMA layer 23 to promotea high MR ratio. Preferably, the magnetic layer 24 is made of CoFeB witha thickness of about 6 to 14 Angstroms so that interfacial perpendicularanisotropy dominates shape anisotropy within the layer to result in PMAwith a magnetization direction that is aligned in the same z-axisdirection as in PMA layer 23. The composite free layer 31 may furtherinclude a Ta insertion layer (not shown) formed between PMA layer 23 andmagnetic layer 24. Seed layer (underlayer) 22 is preferably one of NiCr,NiFeCr, Hf, Hf/NiCr, Hf/NiFeCr, NiFeCr/Hf, or NiCr/Hf with a 5 to 200Angstrom thickness to enhance the PMA character in PMA layer 23 therebyincreasing thermal stability without compromising other free layerproperties.

Referring to FIG. 4, a third embodiment of the present invention isillustrated and depicts a MTJ 50 having a bottom spin valve structurewherein the seed layer/PMA layer of the present invention is employed asa dipole layer to reduce the offset of the minor switching loop of thefree layer caused by a dipole field from the reference layer. Accordingto one embodiment, a seed layer, reference layer, tunnel barrier, freelayer, spacer, dipole layer, and a capping layer are sequentially formedon substrate 21. The seed layer 28 may be NiCr or NiFeCr. Referencelayer 29 and tunnel barrier layer 25 are retained from the secondembodiment. Preferably, the reference layer is thin with a thicknessfrom 5 to 15 Angstroms. Free layer 26 and capping layer 27 werepreviously described with regard to the first embodiment. In one aspect,a CoFeB/MgO/CoFeB reference layer/tunnel barrier/free layer may beemployed to provide a high MR ratio.

A key feature is the stack of layers formed between the free layer andcapping layer. In particular, a non-magnetic spacer 33 made of Ta, forexample, contacts a top surface of free layer 26 and getters oxygen fromthe free layer. Above the spacer is a composite dipole layer 34including an underlayer 22 that contacts a top surface of the spacer anda PMA layer 23 that interfaces with capping layer 27. Layers 22, 23retain the same features as described with respect to the first twoembodiments except the underlayer is preferably 5 to 100 Angstroms thickin this embodiment. In one aspect, free layer 26 may be sufficientlythin (6 to 15 Angstroms) to have significant interfacial perpendicularanisotropy that dominates an in-plane shape anisotropy field such that amagnetization perpendicular to the plane of the free layer isestablished. Interfacial perpendicular anisotropy is a result of theinterface between a bottom surface of free layer 26 and a top surface oftunnel barrier 25 that is preferably MgO. When the free layer has PMA,the magnetization directions of the free layer and PMA layer 23 arepreferably aligned in the same direction.

All of the layers in the MTJ elements described herein may be formed ina sputter deposition system such as an Anelva C-7100 thin filmsputtering system or the like which typically includes three physicalvapor deposition (PVD) chambers each having 5 targets, an oxidationchamber, and a sputter etching chamber. At least one of the PVD chambersis capable of co-sputtering. Typically, the sputter deposition processinvolves an argon sputter gas with ultra-high vacuum and the targets aremade of metal or alloys to be deposited on a substrate. All of the MTJlayers may be formed after a single pump down of the sputter system toenhance throughput.

The present invention also encompasses an annealing step after alllayers in the spin valve structure have been deposited. The MTJ elements20, 40, 50 may be annealed by applying a temperature between 300° C. and400° C. for a period of 30 minutes to 5 hours using a conventional oven,or for only a few seconds when a rapid thermal anneal oven is employed.No applied magnetic field is necessary during the annealing step becausePMA is established in layer 23 due to the seed layer 22 and because ofthe Co—Ni (or A1-A2) spin orbital interactions in the laminated PMAlayer 23.

Once all the layers in MTJ elements 20, 40, or 50 are formed, the spinvalve stack is patterned into an oval, circular, or other shapes from atop-down perspective along the z-axis by a well known photoresistpatterning and reactive ion etch transfer sequence. Thereafter, aninsulation layer (not shown) may be deposited on the substrate 21followed by a planarization step to make the insulation layer coplanarwith the capping layer 27. Next, a top electrode (not shown) may beformed on the capping layer.

Referring to FIG. 5, an embodiment is depicted wherein the MTJ of thefirst embodiment is fabricated in a domain wall motion device. In oneaspect, composite reference layer 30 is formed in a MTJ stack having aseed layer/reference layer/tunnel barrier/free layer/capping layerconfiguration. A key feature is that seed layer 22 and reference layer30 have a width along an in-plane x-axis direction that is substantiallyless than the width of the overlying stack of layers. In fact, the stackof layers including tunnel barrier 25, free layer 26, and capping layer27 may be patterned to provide a wire which from a top-down view (notshown) is part of an array of wires that are employed for storage ofdigital information. Another important feature is that free layer 26 hasa plurality of domain walls (75 a-75 d) each extending vertically from atop surface that interfaces with layer 27 a to a bottom surface whichinterfaces with tunnel barrier layer 25. There is a magnetic domainbounded by each pair of domain walls within the composite free layer.The number of domain walls is variable but is selected as four in theexemplary embodiment for illustrative purposes. In particular, themagnetic domain 92 aligned in a z-axis direction above reference layer30 has a switchable magnetization that changes from a (+) z-direction toa (−) z-direction or vice versa when a switching current is appliedduring a write process. Note that free layer has two ends 37 e, 37 fconnected to a current/voltage source 81 in a first electrical loopincluding wiring 85 a to a junction 82 to wire 83 and to end 37 e, and awire 84 attached to end 37 f to enable a write process. Furthermore,there is a second electrical loop which allows a readout of digitalinformation in the switchable magnetic domain 92 during a read process.Thus, current can be sent from source 81 through wires 85 a, 85 b and toreadout 80 and then to wire 86 and through reference layer 30, tunnelbarrier 25, and free layer 26 before exiting end 37 f and returning tothe source to complete a circuit. In so doing, the readout device 80 isable to recognize whether the switchable magnetic domain 92 has amagnetization in a (+) z-axis direction 90 b or in a (−) z-axisdirection 90 a.

Example 1

An experiment was performed to demonstrate the advantage of a seed layerin improving PMA in an overlying (Co/Ni)_(n) multilayer stack accordingto the present invention. A partial and unpatterned spin valve stackcomprised of a seed layer, a (Co/Ni)₁₀ laminated layer where each Colayer is 2.5 Angstroms thick and each Ni layer is 6 Angstroms thick, anda Ru cap layer was fabricated in order to obtain PMA values from M-Hcurves using a vibrating sample magnometer (VSM). All samples wereannealed at 350° C. for 1 hour. In FIG. 6a , less than ideal seed layerssuch as TaN, PtMn, NiFe, and Ru were employed. In FIG. 6 b, 100 Angstromthick NiCr and Hf seed layers were formed according to the presentinvention to provide improved performance including squarer M-H loopsand higher coercivity (higher PMA) as depicted in curves 41, 42,respectively, compared with the films grown on the inadequate seedlayers in FIG. 6a . Note that a greater distance between the verticalsections of each pair of curves in FIG. 6b means a higher PMA isachieved. Thus, a NiCr seed layer leads to higher PMA than a Hf seedlayer of similar thickness.

Referring to FIGS. 7a-7b , additional partial spin valve stacks wereprepared by sequentially forming a composite seed layer, (Co/Ni)₁₀laminated layer, and Ru capping layer. Coercivity in an in-planedirection with respect to the film plane is illustrated in FIG. 7b for aHf20/NiCr100 seed layer configuration (curve 53) and for a NiCr100/Hf20seed layer configuration (curve 54). Magnetization or PMA in aperpendicular-to-plane direction is represented for the Hf/NiCr andNiCr/Hf configurations in curves 51, 52, respectively in FIG. 7a . Theresulting laminated films grown on composite seed layers of the presentinvention show high coercivity and good squareness. Related measurementson single layer seed films show a high saturation field (perpendicularanisotropy) of about 4000 Oe for a Hf underlayer and around 10000 Oe fora NiCr underlayer. Note that in a composite seed layer such as Hf/NiCr,the upper layer which contacts the Co/Ni (or A1-A2) laminate has alarger effect in enhancing PMA in the multilayer than the lower layercontacting the substrate. Since a NiCr underlayer generates a higher PMAthan a Hf underlayer (10000 Oe vs. 4000 Oe) as indicated previously, aHf/NiCr seed layer (curve 51) leads to a higher PMA than a NiCr/Hf seedlayer (curve 52)

Example 2

To further demonstrate the benefits of the present invention accordingto a first embodiment where the composite seed layer/laminated PMA layeris formed as a reference layer in a MTJ suitable for spin-torque MRAMdevices, a MTJ stack was fabricated as represented in the followingstack of layers where the number following each layer is the thicknessin Angstroms:Si/SiO₂/Ta50/Hf20/NiCr100/(Co2.5/Ni6)₁₀/Co₂₀Fe₆₀B₂₀6/MgO/Co₂₀Fe₆₀B₂₀12/Ta20/Ru.In the aforementioned structure, Si/SiO₂ is the substrate, Ta is abottom electrode, Hf/NiCr is a composite seed layer, (Co/Ni)₁₀ is thelaminated PMA portion of the reference layer, CoFeB is the transitionalmagnetic layer adjoining a MgO tunnel barrier, CoFeB is the free layer,and Ta/Ru is the capping layer. Thus, (Co/Ni)₁₀/Co₂₀Fe₆₀B₂₀ serves as acomposite reference layer to provide a high MR ratio wherein the CoFeBportion is thin enough to preserve the PMA property and thick enough togenerate a high MR ratio because of cohesive tunneling in theCoFeB/MgO/CoFeB stack of layers. In addition to promoting a highmagnetoresistive ratio, the CoFeB free layer is selected for switchingpurposes. An anneal process comprising a 300° C. temperature treatmentfor 1 hour was used for this experiment.

Referring to FIG. 8, a M-H loop measurement is illustrated for the MTJstack described above and shows significant PMA as a result of thecomposite reference layer structure, and an additional PMA contributionfrom the CoFeB free layer. The steps 61 a, 61 b in the M-H loop indicatethe reference layer and free layer switch independently with thereference layer having a much greater coercivity compared with the freelayer as revealed by the greater height for step 61 b related to thereference layer. Therefore, the reference layer may serve as a “pinnedlayer”.

Example 3

According to a third embodiment of the present invention, the compositeseed layer/laminated PMA layer as described previously may beincorporated as a dipole layer in a MTJ represented in the followingstack of layers:Si/SiO₂/Ta50/NiCr100/Co₂₀Fe₆₀B₂₀3/MgO/Co₂₀Fe₆₀B₂₀12/Ta10/NiCr20/(Co2.5/Ni6)₁₀/Ru.In the aforementioned structure, Si/SiO₂ is the substrate, Ta50 is a 50Angstrom thick bottom electrode, NiCr100 is a 100 Angstrom thick seedlayer, CoFeB/MgO/CoFeB is a reference layer/tunnel barrier/free layerconfiguration, and Ta10 is non-magnetic spacer with a 10 Angstromthickness between the free layer and a 20 Angstrom thick NiCr underlayerfor the laminated PMA structure which is (Co/Ni)₁₀. Ru is a cappinglayer.

Referring to FIG. 9, a M-H loop measurement is depicted that showsmagnetization in a direction perpendicular to the film plane of thelayers in the aforementioned MTJ stack having a dipole layer. In thiscase, the dipole layer and free layer both have PMA and switch atdifferent fields allowing the two layers to be set in the desiredconfiguration. Step 71 b for the dipole layer is substantially greaterthan the step 71 a for the free layer. There is no PMA contributionobserved for the reference layer that is purposely kept very thin at 3Angstroms for this experiment. In a functional MTJ, reference layerthickness is generally maintained between 5 and 15 Angstroms and wouldexhibit substantial PMA. By reducing the thickness of the Ta spacerbetween the dipole layer and the free layer, the dipole layer is allowedto provide more offset field to the free layer. Moreover, the thicknessof the NiCr underlayer adjoining the Co/Ni laminate may be adjusted toamplify or reduce the offset field applied to the free layer.

The MTJ elements described herein feature a seed layer/PMA laminatedlayer combination which offers enhanced PMA properties together withimproved compatibility with high temperature processing, and improvedcompatibility with the design and fabrication of magnetic devices. As aresult, the embodiments of the present invention are suitable for avariety of applications including advanced PMA spin-torque MRAM devices,domain wall motion devices, and in-plane magnetic devices wherein it isbeneficial to introduce an out-of-plane magnetic anisotropy component asin in-plane spin torque MRAM devices or in partial PMA spin torque MRAMdevices. Improved PMA properties include increased Hc and Hk that enablehigher thermal stability up to 400° C. processing.

While this invention has been particularly shown and described withreference to, the preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of this invention.

We claim:
 1. A magnetic tunnel junction (MTJ), comprising: (a) a seedlayer formed on a substrate; (b) a composite free layer comprising alower laminated layer that contacts a top surface of the seed layer, thelower laminated layer has intrinsic perpendicular magnetic anisotropy(PMA) and includes two metals, a metal and alloy, or two alloysrepresented by (A1/A2)_(n) or (A1/C/A2) where A1 is a first magneticelement or alloy, A2 is a second magnetic element or alloy, C is anon-magnetic spacer, and n is the number of laminates in the lowerlaminated layer, the composite free layer also includes an uppermagnetic layer that has PMA with a magnetization in the same directionas the PMA in the lower laminated layer; (c) a tunnel barrier layercontacting a top surface of the upper magnetic layer; (d) a referencelayer formed on the tunnel barrier layer; and (e) and a capping layerformed on the reference layer.
 2. The MTJ of claim 1 wherein the seedlayer is Hf, NiFeCr, or NiCr, or a composite with a Hf/NiCr, Hf/NiFeCr,NiFeCr/Hf, or NiCr/Hf configuration and having a thickness between about5 and 100 Angstroms.
 3. The MTJ of claim 1 wherein the upper magneticlayer is CoFeB, CoFe, or combinations thereof with a thickness fromabout 6 and 14 Angstroms.
 4. The MTJ of claim 3 wherein the uppermagnetic layer consists of CoFeB and the composite free layer furthercomprises a Ta layer about 0.5 to 3 Angstroms thick formed between thelower laminated layer and the upper magnetic layer.
 5. The MTJ of claim1 wherein n is from 2 to 30, A1 is a first metal or alloy selected fromone or more of Co, Ni, and Fe that may be doped with boron up to about50 atomic %, A2 is a second metal or alloy selected from one or more ofCo, Fe, Ni, Pt, and Pd, and A1 is unequal to A2.
 6. The MTJ of claim 1wherein C is a Ru layer with a thickness between about 5 and 20Angstroms.
 7. The MTJ of claim 1 wherein the reference layer iscomprised of CoFeB, CoFe, or combinations thereof.